Preheat over to 350 degrees. Spray a 1½ to 2 quart oven proof dish with nonstick spray. Mix the cornbread mix, milk and egg together. Mix in sour cream and butter. Mix in whole corn, creamed corn and onion. Stir until combined. Scrape into prepared dish. Bake for 75 to 105 minutes, until the edges crisp and the center is set. A knife poked into the center doesn’t need to come out clean, but the middle does actually need to be cooked. My mother and grandmother cook this in a water bath like a custard. I find that this increases the cooking time a minimum of a half hour, and I find that I get a better texture baking without one. Experiment and decide what works for you.